The invention relates to a coordinate thermal-cutting machine with a means for automatically setting the initial height of a torch, in particular a plasma cutting torch, above a workpiece surface.
In plasma cutting, a quantity of gas with a high temperature and with a high power density is used; the plasma jet leaves the torch nozzle with such high energy that it is suitable for cutting electrically conducting metal.
"Plasma" is understood to mean a dissociated and partially ionized electrically conducting gas.
The plasma arc for cutting metallic workpieces can only be set up appropriately for the process if the correct igniting distance is maintained between the torch and the workpiece surface. Therefore, it is necessary, before switching on the ignition voltage, to set the cutting torch to an initial height with respect to the workpiece surface which allows the plasma gas to be ignited. Owing to different workpiece thicknesses and surface configurations, this position must be automatically determined in each case.
For setting the initial height of a cutting torch, use is made either of leading electromechanical sensors, which on contact with the workpiece surface emit a stop command to the torch drive, or of proximity switches or approach/analog sensors, which are used contactlessly for switching off the drive.
After igniting the plasma gas, an analog distance sensing system is then further used for distance control in the cutting process or the system switches over to arc-voltage-dependent control.
For cutting processes, until now capacitive or inductive sensors have been used predominantly for distance control. Since these sensors require an opposing workpiece area of at least the same size, they are not suitable for setting the exact initial height of the cutting torches above the workpieces if the sensor is at an edge of the workpiece. In addition, devices with which the cutting torches are protected from being destroyed in the event of a collision are known.
A known means for determining the initial height initially makes the cutting torch approach the workpiece at an adapted approach speed in such a way that it sets down on the workpiece surface. The torch drive is switched off by electrical contact between the tip of the torch and the workpiece or by evaluating the change in the driving force upon setting down. Subsequently, the cutting torch is made to return to the igniting distance by the machine controller (U.S. Pat. No. 4,795,882). In the case of the known means, damage to the cutting torch cannot be ruled out when it is set down on the workpiece. What is more, when working thin sheets, sagging occurs when the cutting torch is set down. The initial height then does not correspond to the unloaded workpiece surface, with the result that the exact igniting distance cannot be maintained. In addition, the unproductive times are increased significantly, since the cutting torch is moved at a low approach speed to the workpiece surface.